CABLE.TXT

How to make a cable to combine the outputs of two drives into one input

The resistor values depend on the rated output impedance of the two
devices and the rated input impedence of the input device.  You may need
to try several values to avoid excessive volume loss if the output
or input impedance is low. 

For normal hi-fi input use 4 each 47Kohm 1/4watt resistors.  For a low
impedence input, the values may be 10K or 560-640 ohms.  For headphone
type impedances, 200-300 ohms.  The resistors are cheap, try one channel
first.  You'll need to either use jacks or cut the 3 cables.  If you're
using a mini-stereo phone cable, the tip is the left channel and the
ring is the right.  The sleeve/shield is common.

Cut the cable, and separate the shield wires from the left and right
center conductors on the two drive output cables, and the input cable.
Connect one resistor to the center conductor of the left cable for one
drive.  Repeat for the other drive cable.  Connect other end of the two 
resistors to each other and to the center conductor of the cable going
to the input device.  Solder the connections.  Tape the wires and 
resistor connections so they cannot touch the other connections or shields.
Repeat for the remaining center conductors on each drive and cable to
the input device.  Connect the shield braid wires of the three cables 
together and solder.    

Test the cable to make sure the volume level is not really low.  If it is,
unwrap and solder a smaller value resistor across each of the originals.
Tape again.  Check.  If okay, tape the braid so it doesn't touch anything
else.  

When the values of the resistors are about correct, the volume loss will
be only about -3db, equal to a volume setting increase of 1/12th rotation.

This type of cable can also be used to mix CD audio with sound card audio
before sending to amplified speakers.

If you need to mix three devices, use a 3rd cable and a resistor from the
center conductor of the 3rd cable, and connect the other end of the
resistor to the junction of the first two before soldering.

Hope this information helps.


You will need to solder 4 ea 47Kohm resistors,
two for each channel of the cable.  For the first channel of the cable,
solder one resistor from the center conductor of the cable to one drive, 
to the center conductor of the cable to the amp/sound card.  Solder a 
second resistor from the center conductor of the second drive to the 
junction of the first and the center
conductor of the cable to the amp/sound card.